Telescopic steering shafts in motor vehicles enable an adjustability of the steering shaft, so that the position of the steering wheel can be adjusted in the longitudinal direction of the steering shaft. Furthermore, in event of a crash the steering shaft can be shoved together, which effectively prevents the steering shaft from moving further into the interior of the passenger compartment and causing injuries to the passengers. This is generally accomplished by providing two mutually telescoping shafts or hollow shafts which together form a steering shaft, which can be shortened or extended appropriately by telescoping relatively movement of the shafts.
U.S. Pat. No. 8,460,116 discloses a roll sliding shaft, consisting of an inner and outer shaft, between which linear ball guideways are arranged. In order to provide the outer ball raceways, the outer shaft of the roll sliding shaft is formed from circular ring segments. This results in a relatively complicated design of a plurality of individual components.
DE10 2008 041 155 A1 proposes an outer tube for a telescopic steering shaft, which is an outer shaft surrounding an inner shaft. In the radial space between the shafts, balls are arranged. These balls roll during the telescoping in parallel with the longitudinal axis of the steering shaft against the outside of the inner shaft and the inside of the outer shaft and thereby ensure an easy adjustability. For this, groovelike ball raceways with cross section in the shape of a circle segment are formed in the outer tube, extending axially in the longitudinal direction. To form the ball raceways, the outer tube comprises a wall thickness varying over the circumference, resulting in a relatively large fabrication cost to generate the cross sectional geometry of the outer tube.
A similarly formed telescopic steering shaft is described in EP 1 693 579 A2. This likewise comprises balls arranged between the mutually telescopic shafts and able to roll in the direction of the longitudinal axis. In this design, the inner and the outer shaft are provided with radially opposite, corresponding ball raceways. The inner shaft is formed as a massive forged piece and the outer shaft comprises a complex cross sectional geometry, similar to that in the aforementioned DE 10 2008 041 155 A1.
A method of making a profiled hollow shaft is known from CH 579 427 A5, in which a hollow shaft is turned about its longitudinal axis during its machining by a roller forming head with ringlike profiled rolls or rollers, so that consecutive single rolling processes by a respective roll are set down next to each other along a helical zone of the hollow shaft surface. In this way, profiled hollow shafts can be made with high precision. The required turning and advancing movement and the rapidly following abrupt single rolling processes, however, make the known method technically complicated, time intensive, and accordingly costly, and therefore not suited to the making of steering shafts.
The drawback of the telescopic steering shafts known in the prior art is the relatively large fabrication expense to form the grooves serving as the ball or rolling body raceways.
Starting from the known prior art, one problem which the present invention proposes to solve is to provide an improved and more economical method for making a profiled hollow shaft for a telescopic steering shaft of a motor vehicle.